[[FIX]] Report invalid syntax as error

This condition was previously reported as a warning. It unambiguously
describes a syntax error, meaning that an error is more appropriate than
a warning. Reporting it as such impoves the interpretation of Test262
tests. Unit tests do not require modification because W116 and E021
share identical messages ("Expected '{a}' and instead saw '{b}'.").

This change triggers failures in four Test262 tests that were previously
interpreted as "valid." These tests concern the "object spread/rest"
proposal, but support for this proposal has not yet been implemented in
JSHint. Therefor, their previous "passing" status was circumstantial,
and the newly-identified error should be marked as "expected."

src/jshint.js

@@ -810,7 +810,7 @@ var JSHINT = (function() {
             relatedToken.line, nextToken.value);
         }
       } else if (nextToken.type !== "(identifier)" || nextToken.value !== expected) {
-        error("W116", nextToken, expected, nextToken.value);
+        error("E021", nextToken, expected, nextToken.value);
       }
     }